#box {
  width: 500px;
  height: 500px;
  margin: 100px auto;
  position: relative;
  overflow: hidden;
  background: #F2F6FC;
  border: 1px solid #DCDFE6;
  border-radius: 6px;
}

#ball {
  width: 10px;
  height: 10px;
  border-radius: 5px;
  position: absolute;
  background: red;
}